A plurality of piping tubes that are used for wrists of paint robots and the like comprise sites at which such tubes are fixed at both ends and brought into contact with other constituents (e.g., other tubes and tube guides). At such sites of fixation, harsh movements such as reciprocal contortion, bending, and scratching are repeated. Thus, tubes used for an apparatus having movement units that undergo flexion, extension, torsion, or friction are required to be durable against such harsh conditions. For example, nylon, polytetrafluoroethylene, or urethane resins have heretofore been known as materials for tubes used for wrists of paint robots (U.S. Pat. No. 6,630,025 (Column 4, lines 5-10); JP Patent Publication (kokai) No. 2004-344789 A (Paragraph 0016); and JP Patent Publication (kokai) No. 2006-150378 A (Paragraph 0028)).
Among such resins, nylon resin has been used as a soft form of nylon comprising a plasticizer. When it is used in a single-layer structure, disadvantageously, plasticizer bleeding takes place with the elapse of time, tubes become hardened or thinned, or cracks or fractures are generated via crazing (i.e., generation of small gaps).
When urethane resin is used in a single-layer structure, swelling, shrinkage, hardening, or other problems arise within a short period of time depending on the solvent atmosphere of the environment surrounding a coating machine due to a lack of solvent tolerance. Thus, use of such urethane resin is almost impossible.
When polytetrafluoroethylene resin is used in a single-layer structure, further, it is extremely poor in terms of anti-flexion-fatigue properties and abrasion tolerance. Thus, cracks or fractures are generated within a short period of time and use of such resin for piping in a robot is almost impossible.
Further, JP Patent Publication (kokai) No. 2003-236425 A (Paragraphs 0027 to 0029) discloses a paint tube with a three-layer structure: i.e., with an innermost layer of polytetrafluoroethylene; an intermediate layer of low-density polyethylene; and an outermost layer that is a conductive sheath. The conductive sheath that constitutes the outermost layer of the paint tube is black in order to impart conductivity. Thus, a fluid such as paint cannot be visually observed from the outside. In addition, a tube with a multi-layer structure is less cost-effective than a tube with a single-layer structure.
If a plastic product has a smooth surface such as a mirror-finish surface, in general, such surface becomes slippery for a fluid (e.g., a liquid or gas) that is in contact therewith, a substance is less likely to adhere thereto, and the coefficient of friction is likely to be lowered. When such plastic product is rubbed against a product of the same type, such products become agglutinated to each other (i.e., adhered to each other), and the slip properties thereof may become poor. Consequently, the coefficient of friction of the product is increased, which leads to increased amounts of abrasion. In extreme cases, defects such as flexion caused by agglutination may disadvantageously occur.
This tendency toward agglutination is likely to occur with another substance such as resin or metal as well as between products of the same type. Because an increased contact area results from a smooth surface, abrasion tolerance deteriorates due to an increase in contact resistance.
Accordingly, a plurality of piping tubes that are used for piping of a fluid (e.g., a liquid or gas) in a paint robot or the like having sites that are in contact with other constituents (e.g., tubes made of the same or different materials or tube guides) at which flexion, shrinkage, torsion, friction, or other movements occur to a significant degree are required to have smooth inner surfaces (i.e., inner circumferential sides), so as to avoid a pressure loss or accumulation of substances. In contrast, a tube with a smaller contact area and lower contact resistance that has an adequately rough (e.g., a satin finish) surface (i.e., the outer surface; the outer circumferential side) would be effective. Such tube is effective in terms of durability at the sites of movement units inside a paint robot or the like where harsh movement is repeated.
The above describes the effects of a technique for finishing the surface of a tube with a single-layer structure in an adequately rough state (e.g., a satin finish) using a monolayer extruder or a method for producing the same. With the use of a multi-layer extruder, for example, molding conditions such as the resin-molding temperature of the inner layer and the outer layer may be altered, so that a tube having an inner surface (the inner circumferential side) with excellent smoothness and an outer surface (the outer circumferential side) with adequate roughness (e.g., a satin finish) while suppressing smoothness can be produced.